Fracture healing of a weight bearing bone is unique in that loading of the injured bone is required for effective healing of the bone. Thus, one day after initial stabilization, optimal rehabilitation requires the patient to use and mechanically load the fractured bone. The most common reason that patients do not fully participate in this needed rehabilitation is that use and loading can cause intense pain. Currently, there is a limited repertoire of analgesics to control skeletal pain (primarily opiates and NSAIDs) and both have significant, unwanted CNS and/or bone healing side effects. The goal of this proposal is to better understand the peripheral nerves that drive fracture pain, determine whether nerves regulate fracture healing and test whether novel therapies reduce fracture pain without unwanted side effects. The hypotheses we will test are;(a) the bone is preferentially innervated by peptidergic C-fibers and A-delta sensory nerve fibers, while lacking innervation by A-beta and non-peptidergic C-fibers, (b) sensory and sympathetic nerve fibers modulate fracture pain and have efferent functions that modulate bone healing, (c) therapies that target the injured and/or sensitized A-delta and/or peptidergic C-fibers will attenuate fracture pain and d) these therapies have beneficial disease modifying actions on bone healing. To test these hypotheses, the fracture pain model will be generated in adult C57BL/6J male mice following closed fracture of the femur. Transgenic mice that endogenously express GFP in restricted populations of nociceptors will be used in combination with immunohistochemistry to determine whether specific subpopulations of sensory nerve fibers innervate the bone and whether the density, distribution and phenotype of nerves fibers that innervate the bone change following fracture. Selective ablation of sensory or sympathetic nerve fibers using capsaicin or guanethidine will allow us to determine whether these nerve fibers regulate bone remodeling/repair/healing and fracture-induced bone pain. Lastly, we will determine whether administration of reference analgesics (morphine, gabapentin) or novel analgesics that regulate peptidergic (a CGRP antagonist, anti-NGF antibody) or non-peptidergic (P2X3 antagonist) nociceptors modifies fracture pain and fracture healing. These studies should provide insight into the neurobiology of fracture pain and bone remodeling and aid in the development of therapies that attenuate fracture pain without the side effects of current analgesics. PUBLIC HEALTH RELEVANCE: Fractures in both young and adults of a load bearing bone frequently induce severe pain interferes with bone healing and functional recovery of the patient. Our goal is to better understand the peripheral nerves that drive fracture pain, determine whether nerves regulate fracture healing and test whether novel therapies reduce fracture pain without unwanted side effects.